This application is related to Japanese Patent Application No. HEI 11(1999)-070299 filed on Mar. 16, 1999, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an organic electroluminescent (hereinafter referred to as EL) device. More particularly, the invention relates to an organic thin-film EL device which can convert electric energy directly to luminous energy when an electric field is applied to its light emitting layer comprised of an organic luminescent substance. The device is usable for a flat light source and a display device.
2. Description of Related Art
With advances of information technology, there has been an increasing need for full color flat panel displays as thinner, less power-consuming and more lightweight display devices than CRTs.
Known as such displays are liquid crystal displays (LCDs) which are not self-luminous, and plasma displays (PDPs) and electroluminescent (EL) displays which are self-luminous.
The EL displays are generally classified, according to their mechanisms of exciting light emission and their constituent materials, into two groups: (1) intrinsic EL devices in which a luminescent substance is excited by local movement of electrons and holes within a light emitting layer and light emission takes place by application of an AC electric field and (2) charge injection type EL devices in which a luminescent substance is excited by recombination of electrons and holes injected from electrodes within a light emitting layer and light emission takes place by application of a DC electric field. For (1) the intrinsic EL devices, inorganic materials are generally used, while for (2) the charge injection type EL devices, organic materials are generally used. That is, the relationship of the intrinsic EL devices being inorganic EL devices and the charge injection type EL devices being organic EL devices holds.
Among these EL devices, the organic EL devices especially are paid remarkable attention to because they have the characteristics of being self-luminous, being able to be designed to consume less power and emitting a wide variety of colors of light.
Conventionally, well known are organic EL devices having a laminated three-layer structure (Appl.Phys.Lett. 56(9) February 1990). An EL device of this kind is constructed to have an anode (positive electrode) formed of an electrode material with a high work function such as indiumxe2x80x94tin oxide (ITO) on the surface of a transparent substrate of glass or the like; a hole transporting layer, a light emitting layer and an electron transporting layer which are organic layers formed on the anode; and further a cathode (negative electrode) formed of an electrode material with a low work function such as Mg/Ag on the top. By applying voltage across both the electrodes of this organic EL device or by passing electric current across the electrodes, electrons and holes are injected into the light emitting layer. Energy generated by recombination of electron-hole pairs within the light emitting layer is released as fluorescence or phosphorescence. Light is thus emitted. Accordingly, light of optional colors, blue to red, can be obtained by using luminescent materials having fluorescence of blue to red for the light emitting layer.
In the organic EL devices, the charge injection type EL devices, effective injection of charges from the electrodes (i.e., holes from the anode and electrons from the cathode) to the organic layers is important from the viewpoint of reducing power consumption. Usually, electrons injected from the cathode fall short of holes injected from the anode. For this reason, to improve efficiency in injecting electrons from the cathode (hereinafter referred to as electron injection efficiency) is essential for low-voltage driving of EL devices and enhanced efficiency thereof.
In order to raise the electron injection efficiency,
(1) using a cathode material which provides a higher electron injection efficiency,
(2) using, as the cathode, a laminated conductive layer of a metal having a low work function and a stable metal,
(3) using, as the cathode, a mixture electrode of a metal having a small work function and a stable metal,
(4) providing a buffer layer (an electron injection layer) between the cathode and the organic layers,
(5) providing a co-deposited layer between the cathode and the organic layers,
and the like have been conventionally studied.
Typical examples of (1) include a calcium cathode (Nature, 347, 539 541) and the like. Typical examples of (2) include a layered cathode of Mg and Ag (Appl.Phys.Lett., 59(21), Nov. 18, 1991) and the like. Typical examples of (3) include a co-deposited cathode of Mg and Ag (Appl.Phys.Lett., 51(12), Sep. 21, 1987), co-deposited cathode of Li and Al (Synthetic Materials, 91(1997)129-130) and the like. Typical examples of (4) include an electron injection layer of dithienosilole (The 59th Autumn Meeting, 1998; the Japan Society of Applied Physics, 16p-YH-2) and the like. Typical examples of (5) include a co-deposited layer of Alq3/Mg between an organic layer of Alq3 and an electrode of Mg (The 59th Autumn Meeting, 1998; the Japan Society of Applied Physics, 16a-YH-7).
However, according to (1), (2) and (3), since Ca, Mg and Li used in cathodes react with moisture to produce hydroxides, there arises a problem in stability. In addition, according to (3), it is difficult to produce mixture electrodes with maintaining a constant ratio of the two metals, and therefore, it is hard to obtain cathodes having stable properties. As for (4) and (5), there is a problem in that the electron injection efficiency is poorer compared with the other three.
In these circumstances, Han et al. (Japanese Unexamined Patent Publication No. HEI 10(1998)-74586 and Fujikawa et al. (The 59th Autumn Meeting, 1998; the Japan Society of Applied Physics, 16a-YH-7) propose cathodes having a combined structure of an insulating layer of a fluoride and a conductive layer of a metal.
However, even though such a cathode composed of a metal and an insulator of a fluoride is used for an organic EL device, sufficient injection of electrons into the organic layers has not been achieved yet.
The present invention provides an organic electroluminescent device comprising: a pair of an anode and cathode, a single-layered or multi-layered film therebetween containing an organic luminescent material and an insulating film of a chloride, a bromide or an iodide between the cathode and the single-layered or multi-layered organic film, the device being adapted to emit light by application of voltage or current across the pair of electrodes.
In other words, as a result of concentrated studies of the buffer layer, materials for the cathode and the like for improving the electron injection efficiency, the inventors of the present invention have improved the electron injection efficiency by providing an insulating layer of a cation metal having a low work function between the cathode and the layer of an organic luminescent material, and consequently have realized an organic EL device which can be driven with a low voltage, finally to achieve the present invention.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.